条件运算符？(conditional Operator ? : type evaluation)

条件运算符？(conditional Operator ? : type evaluation) void method(Set<String> whiteListProviders){ HashSet<String> hashedWhitelistedProviders; HashSet<String> fdsfh = (hashedWhitelistedProviders = (HashSet<String>) whitelistedProviders); HashSet<String> ghjk = (hashedWhitelistedProviders = new HashSet<String>(whitelistedProviders)); HashSet<String> gh4jk = true ? fdsfh : ghjk; //compiles true?fdsfh:ghjk; //gives error "Type mismatch: cannot convert from HashSet<String> to boolean" }

我读了http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.25，但仍然无法理解为什么它在eclipse中给出了编译错误

void method(Set<String> whiteListProviders){ HashSet<String> hashedWhitelistedProviders; HashSet<String> fdsfh = (hashedWhitelistedProviders = (HashSet<String>) whitelistedProviders); HashSet<String> ghjk = (hashedWhitelistedProviders = new HashSet<String>(whitelistedProviders)); HashSet<String> gh4jk = true ? fdsfh : ghjk; //compiles true?fdsfh:ghjk; //gives error "Type mismatch: cannot convert from HashSet<String> to boolean" }

I read http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.25 but still couldn't understand why it is giving compilation error in eclipse

最满意答案

Java只允许在需要语句的地方进行赋值和调用，而不是任意表达式。

从JLS第14.8节：

某些类型的表达式可以用分号跟随它们作为语句：

ExpressionStatement: StatementExpression ; StatementExpression: Assignment PreIncrementExpression PreDecrementExpression PostIncrementExpression PostDecrementExpression MethodInvocation ClassInstanceCreationExpression

三元运算符（ ConditionalExpression ）不在该列表中，因此除了作为较大表达式或初始值设定项的一部分之外，它不会出现。

Java only allows assignments and calls where a statement is expected, not arbitrary expressions.

From section 14.8 of the JLS:

Certain kinds of expressions may be used as statements by following them with semicolons:

ExpressionStatement: StatementExpression ; StatementExpression: Assignment PreIncrementExpression PreDecrementExpression PostIncrementExpression PostDecrementExpression MethodInvocation ClassInstanceCreationExpression

The ternary operator (ConditionalExpression) is not on that list, so it can't appear except as part of a larger expression or initializer.